A new study suggests that environment accounts for more than half of autism risk, while genes are responsible for about 40 percent.

In their quest to decipher the complex code of autism, which can run in families, most scientists have focused on genes. But a large survey of identical and fraternal twins, published in July, offers convincing evidence that environmental factors are at least as crucial as genetics in determining whether a child will develop the neuropsychiatric disease. “I think the environment has to be taken seriously,” says lead author Joachim Hallmayer, a psychiatric geneticist at the Stanford University School of Medicine. “There is a huge role for other factors besides genetics that we do not understand.”

Twin studies are valuable because they help distinguish between genetic and environmental contributors. Identical twins share all their genes; fraternal twins share no more genes than normal siblings do, but they get exposed to the same environment in the womb and at home during infancy. Past twin studies of autism had flaws, however. A 1977 study concluded autism was almost entirely the result of genetics, but it included only 21 twin pairs and came at a time when diagnostic tests for the disorder were poor.

Hallmayer’s study of 192 twin pairs tells a much different story. As expected, he found that if one identical twin has autism, the other usually does too. However, he also saw higher rates for fraternal twins—31 percent for males and 36 percent for females—than genetics alone would predict. A statistical analysis showed that environmental factors accounted for more than half of autism risk. Genes were about 40 percent responsible.

Now researchers face the challenge of identifying specific environmental triggers. Vaccinations have long been ruled out, but this year studies of influences in the womb have tentatively linked mothers’ antidepressant use, high blood pressure, and diabetes to increased autism rates. Scientists are eagerly awaiting results from a near-complete study examining autism’s link to mothers’ immune cells that attack brain proteins in the developing fetus.